High power laser diode

ABSTRACT

The high power laser diode comprise an active region and a trapping region separated by a passive region, all situated between a p-type confinement region and an n type confinement region such that the radiation field is attracted toward and spread into the trapping region assuring a reduced value for the confinement factor into the active region.

FIELD OF THE INVENTION

The invention describes a high power laser diode with potential use inophthalmology, surgery, printing, magneto-optical data storage.

BACKGROUND OF THE INVENTION

There are already known high power laser diodes made of symmetricalstructures with separate confinement in which the light is emitted in avery narrow active region and is guided by a larger wave guide. Thesediodes have the following disadvantages:

they are optimized for low threshold current and as a consequence theyhave an as high as possible confinement factor (the ratio of the powerdensity through the active region to the total power density) and thatinduces the catastrophical degradation of the mirror at relative smallpower densities and allows the optimal operation only for short devices,due to the high value of the modal gain;

they have reduced operational safety due to the low value of the activeregion thickness;

they are not optimized for maximum power densities, relative to theemitting stripe width, optimization for which two conditions have to befulfilled: a low value of the confinement factor (<0.01) together with ahigh value of the active region thickness (>10 nm).

There are already known high power laser diodes structures with lowconfinement factor, in the range 0.0015 and 0.00015 (patent RO 102871,PCT Application RO 91/0002), consisting of a main region of the waveguide (2-5 μm thickness) and of an active region, lateral to the mainregion and separated from it by an intermediate region. The radiationpropagates mainly in the main region, which is very large, and thecut-off of the high-order modes is realized by a very small step of therefractive index value between the confinement region and the mainregion. They have the following disadvantages:

the imposed step of the refractive index between the main region and theconfinement region is very small and very difficult to be realized fromthe technological point of view,

the passive region confinement factor value is almost equal to unity andits band gap is very close to the energy of the propagating photons,conditions which together can enhance two photon absorption.

SUMMARY OF THE INVENTION

The problem solved by this invention is the designing of a laser diodestructure made of multiple layers which operates with a low value of theconfinement factor less, than 0.015, and which has the thickness of theactive region greater than 0.01 μm and small values for the confinementfactor of the small energy gap layers, others than the active region.

The high power laser diode structures, corresponding to this invention,eliminate the disadvantages of the other known solutions since they arecomprised of an n-type confinement region; a p-type confinement regionwhich has the lowest value of the refractive index in the structure; apassive region situated between the two confinement regions whichrepresents the main part of the wave guide and whose refractive indexmay be lowered stepwise or continuously relative to the refractive indexof the n-type confinement region, this decrease being greater at(towards) the limit of separation between the passive region and thep-type confinement region; a thin active region situated asymmetricallyin the passive region and closer to the p-type confinement region andwhose refractive index is much greater than the refractive index of thepassive region; a thin balance region situated at the limit between thepassive region and the n-type confinement region, which balance theaction of the active region and whose refractive index is much higherthan the refractive index of its neighbor regions, i.e. the passiveregion and the n-type confinement region; the structure being such thatthe refractive index profile has two marked bumps, one corresponding tothe active region and the other corresponding to the balance region,both bumps having the magnitude less than λ/4. due to the very smallthickness of these two regions; and such that the energy band gap of thebalance region is much greater than the energy band gap of the activeregion to avoid the absorption in the balance region of the radiationemitted in the active region; and since, by the combined action of twofactors, the prevalent attraction of the field by the balance region ascompared with the attraction of the active region and the repellingtoward the n-type confinement region of the field due to the refractiveindex step at the margin between the p-type confinement region and thepassive region, the named laser diode structures assures a fielddistribution with its maximum situated in the balance region, outsidethe active region bump, and a reduced value for the confinement factorof the active region by repelling the field distribution maximum fromthe active region, together with a reduced value of the confinementfactor of the balance region due to its low thickness.

The high power laser diodes according to the invention present thefollowing advantages:

they have a reduced confinement factor, lower than 0.015, although theactive region is rather thick, thicker than 10 nm;

they operate in the fundamental transversal mode;

they have low confinement factor for low band gap regions others thanthe active region;

they allow the fabrication of stripes with low modal gain.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, other features of the diodes according to theinvention will become apparent from the specific description andexamples, in connection with FIGS. 1-7 which represent:

FIG. 1: a perspective view of the laser diodes according to theinvention

FIG. 2: a refractive index profile along the direction perpendicular tothe wave guide layers

FIG. 3: the field distribution of the structure in FIG. 2

FIG. 4: the energy band gap profile for the structure in FIG. 2

FIG. 5: a refractive index profile for a structure with two balanceregions

FIG. 6: the field distribution for the structure in FIG. 5

FIG. 7: a transversal section through a structure with a stripe

DESCRIPTION OF THE PREFERED EMBODIMENT

The high power laser diode structure according to the invention, andrelated to FIG. 1, consists of the following regions: a substrate 1, ann-type confinement region 2, a p-type confinement region 3, a passiveregion 4, situated between the two named confinement regions and whichconstitutes the main part of the wave guide, an active region 5,situated inside the wave guide and closer to the p-type confinementregion, a balance region 6 which equilibrates the active regioninfluence, situated inside the wave guide at the separation limitbetween the passive region and the n-type confinement region, a contactlayer 7, a p-type metallic contact 8, and a n-type metallic contact 9.Because in some cases it is possible for the active region to be notalways situated at the limit between the passive region and the p-typeconfinement layer, in these cases the passive region is divided into twoparts: a part 4, between the balance region 6 and the active region 5,and a part 4′, between the active region 5 and the p-type confinementregion 3. With reference to an orthogonal coordinate system Ozyx, theregions interfaces are parallel to each other and also parallel with theplane yOz, and the laser radiation is propagating in the Oz direction.The laser radiation is produced in the active region by the injection ofthe minority carriers in a p-n junction situated inside or in theneighborhood of the active region. The injection current is produced byapplying a positive bias between the two metallic contacts 8 and 9.

In each layer propagates a power flux P₁ . . . P₇ so that the totalpower is P=ΣP₁. For each region a confinement factor Γ₁ . . . Γ₇ can bedefined, and by definition Γ₁=P₁/P.

The refraction index varies in the Ox direction. The refraction indexesfor the regions 1 . . . 7 have correspondingly the symbols n₁ . . . n₇.The profile of the refractive index along the Ox direction for a typicalstructure is presented in FIG. 2. The field distribution in thisstructure in presented in FIG. 3. The profile of the energy band gap ofthe regions along the Ox direction is presented in FIG. 4. Therefractive index profile has two bumps: a bump 10 corresponding toactive region and a bump 11 corresponding to the balance region. Theactive region 5 is situated near the p-type confinement layer 3, and thebalance region 6 is situated at the separation limit between the passiveregion 4 and n-type confinement region 2. The thickness of the activeregion and of the balance region are d₅ and d₆, respectively, and thethickness of the passive region including d₅ and d₆, is d₄. Thedistances between the balance region and the active region, and betweenthe active region and the p-type confinement region, are I₄ and I′₄,respectively. The refractive index of the active region n₅ has thehighest value. This high value is a consequence of a low value of theenergy band gap of the active region. The energy band gap of the passiveregion is higher, in order to obtain an efficient carrier confinementinside the active region. Corresponding to this higher value of theenergy band gap, the refractive index in the passive region n₄, is lowerthan n₅. The difference of the refractive indexes (n₅−n₄) determines,together with the effective thickness of the active region d₅, themagnitude of a bump in the refractive index profile, magnitude equal to:

d ₅ (n ₅ ² −n ₄ ²)^(½)

To avoid the confinement of the radiation field inside the activeregion, the magnitude of the bump of the active region must be lowerthan λ/4, i.e. the following condition has to be fulfilled:

d ₅ (₅ ² −n ₄ ²)^(½)∴λ/4  (1)

The lowest value of the refractive index is that of the p-typeconfinement region, n₃. The next higher value is that of the refractiveindex of the passive region n₄, then that of the refractive index of then-type confinement region, n₂. A high difference between n₄ and n₃,induces the repelling of the optical field toward the n-type confinementregion.

The balance region 6 has the role to attract toward it the maximum ofthe optical field distribution, thus reducing the value of theconfinement factor. The refractive index of this region, n₆, is higherthan the refractive index of the adjacent regions: the passive regionand the n-type confinement region. The attraction of the optical fieldtoward the bump of the balance region is enhanced by the repelling ofthe optical field due to the existence of a high variation of therefractive index at the limit between the passive region and p-typeconfinement region to make the action of the balance region moreefficient, the magnitude of the its bump must be as high as possible,i.e. the difference n₆−n₂ must be as high as possible. On the other handn₆ can not be very close to ns because this would determine the energyband gap of the active region and the balance region to be also veryclose and this would induce the balance region absorption of theradiation emitted in the active region. Although the efficiency of thebump of the balance region is direct proportional with its magnitude,the magnitude of the bump of the balance region must not be very closeto the λ/4 value, since in this case the optical field would be capturedentirely inside the balance region, Γ₆ would become almost unity, andthe two photons absorption processes in a region with a relative narrowenergy band gap would be predominant compared with the other absorptionprocesses.

To obtain low values of the confinement factor Γ₅, the active regionmust be as far as possible from the middle of the wave guide. At thelimit it will be near the p-type confinement region and I′₄=0. Thedistance between the active region and the balance region is I₄.

The higher order modes are cutt off by imposing the conditions that therefractive index of the n-type confinement region is higher or equalthen the refractive index of the passive region and that the two bumpmagnitudes are lower than λ/4. To avoid the higher order modes, a small(positive) difference between the refractive index of the n-typeconfinement region and the refractive index of the passive region shouldbe used, especially in the case when the two bumps, both of highmagnitude, are far from one another.

To illustrate the combined action, the repelling of the optical fielddue to the difference between the values of the refractive index of thepassive region and the p-type confinement region, on one hand, and theattraction of the optical field toward the balance region, on the otherhand, we will first analyze two structures differing by the thickness ofthe passive region. Both structures have the thickness of the activeregion d₅=80 nm and the thickness of the balance region d₆=60 nm. It isobvious that the bump of the balance region is lower than the bump ofthe active region, if one takes in account their thickness and the factthat the refractive index of the balance region is lower than therefractive index of the active region. Both structures consists ofmultiple regions from the system Al_(x)Ga_(1−x)As, with the followingcomposition indexes: the p-type confinement region 3 with x₃=0.6, nextto it the active region 5 with x₅=0.0, then the passive region 4 withx₄=0.35, the balance region 6 with x₆=0.15, and the n-type confinementregion 2 with x₂=0.332. The refractive indexes are determined by thecorresponding composition indexes (the refractive indexes decrease whenthe composition indexes increase). If the first analyzed structure wouldhave the thickness of the passive region 4, d₄=0.88 μm then it has aconfinement factor of the active region Γ₅=0.0147 and a confinementfactor of the balance region Γ₆=0.053. If for the second structure thethickness of the passive region 4 would be increased at d₄=1.45 μm then,the two above mentioned confinement factor decrease to Γ₅=0.00165 andΓ₆=0.0456, respectively.

An other example is illustrated by FIG. 2. The p-type confinement region3 has, as in the above mentioned example, a composition index x₃=0.6.The passive region 4 and the n-type confinement region 2 have equalcomposition indexes x₄=x₂=0.35. The active region 5, with x₅=0.0 andd₅=20 nm, is separated from the p-type confinement region 3 at adistance I′₄=0.18 μm, and the balance region 6 with x₆=0.15 and d₆=100nm is separated from the active region 5 at a distance I₄=0.4 μm. Thisstructure has a confinement factor of the active region Γ₅=0.0072 and aconfinement factor of the balance region Γ₆=0.193.

In the following, other examples of structures with low confinementfactor and with relatively high thickness of the active region, in therange 10 . . . 80 nm will be presented. In these examples the thicknessof the balance region was chosen to be 100 nm. The fact that thethickness of the balance region is higher compared to values of theabove first presented examples, determines a higher value for theconfinement factor of this region, a possible enhancement of thetwo-photon absorption effects and a contraction of the width of thefield distribution. The values chosen as examples for the confinementfactor are 0.0144, 0.0072, 0.0036, 0.0018. The values chosen as examplesfor the thickness of the active region are 10 nm, 20 nm, 40 nm and 80nm. The case for the 10 nm active region thickness wood need somecorrection for the values of the refractive indexes of the structurecorresponding to the quantum well effects. This corrections are notessential for the demonstration and are not taken into account.

In all following examples the active region is adjacent to the p-typeconfinement region, thus that I′₄=0.0. This selection simplifies theprocedures for the stripe fabrication, which will be discussed later.

In the first set of examples, those from table 1, the structures willconsists of semiconductor materials from the Al_(x)Ga_(1−x)As systemwith the composition indexes x₄=0.35, x₅=0.0, x₆=0.15. The compositionindex of the p-type confinement region is x₃=0.35 for the case d₅10 nm,is x₃=0.40 for the case d₅=20 nm, is x₃=0.48 case d₅=40 nm and isx₃=0.60 for the case d₅=80 nm. The increase in the composition index ofthe p-type confinement region is determined by the necessity of an, asefficient as possible, repelling action of the optical field from thep-type confinement region. The composition index of the n-typeconfinement region 2, is x₂=x₄ for the most cases, except for the caseswhere the thickness of the active region is 80 nm, when differences ofthe composition indexes between the n-type confinement region and thepassive region have been introduced to cut-off the higher order modes.The examples shall be given for a active region made up of a single‘thick’ layer. Small differences may appear if a few quantum-wellsreplace a single equivalent ‘thick’ layer.

TABLE 1 No. d₅ (nm) Γ₅ x₂ x₃ l₄ (μm) Γ₆ 1 10 0.0144 0.35 0.35 0.08 0.1952 10 0.0072 0.35 0.35 0.255 0.181 3 10 0.0036 0.35 0.35 0.44 0.179 4 100.0018 0.35 0.35 0.62 0.180 5 20 0.0144 0.35 0.40 0.23 0.199 6 20 0.00720.35 0.40 0.38 0.192 7 20 0.0036 0.35 0.40 0.55 0.188 8 20 0.0018 0.350.40 0.70 0.187 9 40 0.0144 0.35 0.48 0.40 0.191 10 40 0.0072 0.35 0.480.57 0.187 11 40 0.0036 0.35 0.48 0.74 0.186 12 40 0.0018 0.35 0.48 0.900.186 13 80 0.0144 0.335 0.60 0.77 0.163 14 80 0.0072 0.330 0.60 0.860.161 15 80 0.0036 0.325 0.60 0.93 0.154 16 80 0.0018 0.320 0.60 0.980.143

Analyzing the table, one can see that there are a few possibilities toreduce the confinement factor:

the reduction of the refractive index in the p-type confinement region(sequence 13 . . . 16 compared to sequence 9 . . . 12 etc.)

the enlargement of the distance between the active region and thebalance region (inside each sequence 1 . . . 4, 5 . . . 8, 9 . . . 12,13 . . . 16)

the increase of the refractive index in the n-type confinement regioncompared to the refractive index in the passive region (examples 13 . .. 16)

Beside these possibilities there is also the possibility to increase thedistance between the active region and the p-type confinement region andto reduce in this way the repelling action for the optical field, and soto enhance the confinement factor.

TABLE 2 No. d₅ (nm) Γ₅ x₂ x₃ l₄ (μm) Γ₆ 1 10 0.0144 0.35 0.35 0.09 0.2062 10 0.0072 0.35 0.35 0.26 0.191 3 10 0.0036 0.35 0.35 0.44 0.189 4 100.0018 0.35 0.35 0.60 0.191 5 20 0.0144 0.35 0.40 0.22 0.213 6 20 0.00720.35 0.40 0.36 0.205 7 20 0.0036 0.35 0.40 0.51 0.201 8 20 0.0018 0.350.40 0.67 0.199 9 40 0.0144 0.35 0.48 0.40 0.201 10 40 0.0072 0.35 0.480.56 0.198 11 40 0.0036 0.35 0.48 0.71 0.197 12 40 0.0018 0.35 0.48 0.860.197 13 80 0.0144 0.335 0.60 0.60 0.167 14 80 0.0072 0.330 0.60 0.600.161 15 80 0.0036 0.325 0.60 0.60 0.154 16 80 0.0018 0.320 0.60 0.600.143

In table 2 examples of laser diode structures with an other set ofcomposition indexes are shown, differing from the one used for table 1,so that the composition index of the active region is 0.11 and thewavelength of the emitted light is close to 800 nm. The compositionindexes for regions 4, 5, 6 are: 0.35, 0.11 and 0.21, respectively, andthe composition indexes for the p-type confinement region are 0.35,0.40, 0.48 and 0.60 for active region thickness of 10 nm, 20 nm, 40 nmand 80 nm, respectively. The composition index in the n-type confinementlayer 2 is given in the table for each case separately. The refractiveindex in the n-type confinement region step up compared to therefractive index of the passive region will be exploited to cut-off thehigher order modes in structures with the thickest active region

We consider that the concepts shown in this invention and the examplesgiven for structures in the Al_(x)Ga_(1−x)As system, having constantcomposition index layers, can be extended to other cases too:

materials from the InGaAs, InGaAsP, InGaAlAs systems, and other largeenergy band gap materials, with an as low as possible probability forthe two-photon processes;

structures with continuous gradual (not stepwise) variation of therefractive index;

An other extension of the invention refers to the number of balanceregions. Beside the balance region 6 at the limit of the passive region4 and the n-type confinement region 2, one or more secondary balanceregions, 6′, 6″ etc. can be introduced between the balance region 6 andthe active region 5, with the value of the bumps 11′, 11″ etc. alsolower than λ/4. In the following we shall give an example of a structurewith a secondary balance region having the following properties: thecomposition indexes for the p-type confinement region 3, the passiveregion 4 the active region 5, the secondary balance region 6′, the mainbalance region 6 and the n-type confinement region 2 are: 0.6, 0.35,0.0, 0.15, 0.15 and 0.32, respectively. The thickness of the activeregion 5, the secondary balance region 6′ and the main balance region 6are 80 nm, 60 nm and 80 nm respectively. The active region is placed atthe limit between the p-type confinement region 3 and the passive region4. The distance between the active region 5 and the secondary balanceregion 6′ and the distance between the secondary balance region 6′ andthe main balance region 6 are both equal to 0.6 μm. This structure has aconfinement factor of 1.87 10⁻³. The refractive index profile of thisstructure is presented in FIG. 5. The optical field distribution forthis structure is presented in FIG. 6.

The above described structures can be obtained by epitaxyally growth,with thin layers, parallel each another and to the growth substrate.Stripes with a width w of several microns are defined from thesestructures by various methods. The demarcation of the stripes has twopurposes:

the limitation of the injection current only inside stripes,

the build up of a refractive index variation from the stripe to theadjacent lateral regions, to restrict the optical field inside thestripe.

A very common used method to define the stripes is the creation of aridge, by partial etching of the p-type confinement regions and by thesubsequent oxidation of the regions where the etching was performed.This ridge is covered by a metal contact and through it the electriccurrent will flow. The thinning of the p-type confinement regionproduces a small decrease of the effective refractive index which isused for the (partial) confinement of the optical field. Thedisadvantage of this method resides in the fact that the active regionis not removed outside the stripe and that its lateral regions that arenot excited by the electric current are strongly absorbant, thatenhances the modal attenuation coefficient.

In FIG. 7 a section through a stripe structure is presented, whichderives from a dual planar structure, with two bumps of the refractiveindex profile in the Ox-direction of the materials which forms theplanar structure. The stripe has a width w. Along the w width, theeptaxial structure contains all the regions described in FIG. 2. On bothsides of the stripe, the epitaxial structure contains a i-typeconfinement region 12 and does not contain the active region 5 and thep-type confinement region 3. The i-type regions 12 can consists of asemiconductor material, from the same material system as the rest ofmaterials from which the stripe structure is made, and can be insulatingfrom electrical point of view. In the case of the Al_(x)Ga_(1−x)Assystem materials, their refractive index can be determined by theircomposition index. In the stripe zone, the wave guide of width w isformed mainly around the bump of balance region, and is laterallybordered by wave guides formed mainly around the same bump of thebalance region, not containing the active region.

From the adjustment of the refractive index of the regions 12 one canobtain a fine tuning of the effective refractive indexes in the stripewave guide and in the lateral wave guides, such that the variations ofthe effective refractive indexes to permit the operation in the lateralfundamental mode. Thus, for the structures number 6, 10 and 14 fromtable 1, having all a confinement factor of 0.0072, by eliminating thep-type confinement region 3 and the active region 5 and substitutingthem with i-type confinement regions 12 having a composition indexx₁₂=0.35, the variation of the effective refractive indexes are equal to0.0003, i.e. the optimum value for stripes of 12 μm width, whichcorresponds to the chosen confinement factor [1].

We claim:
 1. A high power laser diode comprising a substrate and astructure that acts as a waveguide for a radiation field, said structureincluding: an n-type confinement region (2); a p-type confinement region(3) which has the lowest value of the refractive index in the structure;a first passive region (4) situated between the two confinement regions(2) and (3), which represents the main part of the wave guide and whoserefractive index is intermediate between the refractive index of thep-type confinement region (3) and the refractive index of the n-typeconfinement region (2); the refractive index of the first passive region(4) has a monotonous trend from one of its ends toward the other; anactive region (5) situated between the p-type confinement region (3) andthe passive region (4), whose refractive index is much higher that therefractive index of the passive region (4); a trapping region (6),situated between the passive region (4) and the n-type confinementregion (2), which balance the attraction of the radiation field by theactive region (5) and whose refractive index is higher than therefractive index of its neighbor regions, i.e. the passive region (4)and n-type confinement region (2); the structure being such that: therefractive index profile has two marked bumps, one corresponding to theactive region (5) and the other corresponding to the trapping region(6); the energy band-gap of the trapping region (6) is much greater thanthe energy band-gap of the active region (5) to avoid the absorption inthe trapping region (6) of the radiation emitted in the active region(5); the attraction of the radiation field by the trapping region (6)and the repelling of the radiation field by the refractive index whichincreases from an interior margin of the p-type confinement region (3)toward the n type confinement region (2) both assure a reduced value forthe confinement factor of the active region (5).
 2. A high power laserdiode according to claim 1, comprising at least one secondary trappingregions placed between the active region (5) and the main trappingregion (6).
 3. A high power laser diode according to claim 1, comprisinga stripe structure, and on each side of the stripe the epitaxiallystructure contains a confinement region (12) and does not contain theactive region (5) and the p-type confinement region (3), so that in thearea of the stripe, the wave guide of width w is formed mainly around abump of the trapping region and is restricted laterally by wave guidesformed mainly around the same bump of the trapping region.
 4. A highpower laser diode according to claim 1 comprising a second passiveregion (4′) situated between the p-type confinement region (3) and theactive region (5); the second passive region having a refractive indexintermediate between the refractive index of the p-type confinementregion and the refractive index of the first passive region; and, therefractive index of the second passive region (4′) having a monotonoustrend from one of its ends toward the other.
 5. A high power laser diodeaccording to claim 1 wherein the passive region has a constant value. 6.A high power laser diode according to claim 1 wherein the p-typeconfinement region refractive index is equal to the n-type confinementregion refractive index and also equal to the refractive index of thepassive region.
 7. A high power laser diode according to claim 1 whereinthe active region bump has the property that a thickness of the activeregion (5) multiplied by the square root of the difference between theactive region refractive index squared and adjacent passive regionrefractive index squared is less than λ/4; and the trapping region bumphas the property that the trapping region thickness multiplied by thesquare root of the difference between the trapping region refractiveindex squared and the n-type confinement region refractive index squaredis less than λ/4.
 8. A high power laser diode according to claim 2wherein the passive region has a constant value.
 9. A high power laserdiode according to claim 2 wherein the p-type confinement regionrefractive index is equal to the n-type confinement region refractiveindex and also equal to refractive index of the passive region.